Method for electrostatic discharge protection in integrated circuits

ABSTRACT

An output circuit of an integrated circuit device includes first and second MOS transistors including respective spaced apart pairs of source and drain regions in a substrate, arranged such that respective first and second channels of the first and second MOS transistors are laterally displaced with respect to one another. The output circuit further includes an isolation region in the substrate, disposed between the first and second MOS transistors. A first conductor connects the source region of the first MOS transistor to a power supply node. A second conductor connects the drain region of the first MOS transistor to the source region of the second MOS transistor. A third conductor connects the drain region of the second MOS transistor to an external signal pad of the integrated circuit device. The isolation region may comprise first and second insulation regions surrounding respective ones of the first and second MOS transistors, and a guard ring surrounding and separating the insulation regions.

RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.10/252,631 filed on Sep. 23, 2002 now U.S. Pat. No. 6,914,305, whichclaims priority to Korean Patent Application No. 2001-59954, filed onSep. 27, 2001, the contents of which are hereby incorporated byreference in their entirety.

FIELD OF THE INVENTION

The present invention relates to integrated circuit devices, moreparticularly, to circuits and methods for protecting integrated circuitsfrom electrostatic discharge.

BACKGROUND OF THE INVENTION

A thrust of integrated circuit design has been the development ofintegrated circuits capable of higher frequency operation and/or lowerpower consumption. The ability of an integrated circuit to operate athigh frequencies with low power consumption is generally determined bycharacteristics of active and passive elements in the integratedcircuit, such as resistance and parasitic capacitance.

Referring to FIGS. 1A, 1B, and 1C, in order to reduce the drain andsource resistances Rd, Rs of an NMOS transistor 10′ or a PMOS transistor10″, silicide layers SA comprising a low-resistance metallic materialand silicon are formed on surfaces of a polysilicon gate GP, a source S,and a drain D using a self-aligned silicidation (hereinafter“salicidation”) process. In applying the salicidation process, adischarge space for the transistor is defined at a region A at ajunction disposed under a spacer GS adjacent the polysilicon gate GP.When electrical transients arising from electrostatic discharge (ESD) orelectrical overstress (EOS) occur at a pad (not shown) connected to oneof the source S or the drain D, the discharge space A may not besufficiently large enough to prevent physical damage.

Input/output circuits are commonly designed to protect internal portionsof an integrated circuit from transients arising from ESD, EOS, peakvoltage, current surge, or noise. They also commonly provide voltageconversion between the voltage used by the internal portions and thevoltage used by externally connected circuits, e.g., conversion ofsignals from CMOS to TTL or from TTL to CMOS. Input/output circuits alsooften include transistors with larger channel widths that can supporthigher currents which may arise from resistance, inductance, andcapacitance of a printed circuit board (PCB) on which the integratedcircuit is mounted and cables connecting the integrated circuit toexternal systems.

For example, a structure, as shown in FIG. 2A, that includes multipletransistors including a plurality of polysilicon gate layers 3 formed ona diffusion region 1 including source and drain regions S, D may beused. As can be seen in the cross-section in FIG. 2B, the substrateP-sub and the source and drain regions S, D form parasitic horizontalNPN bipolar transistors Q1, Q2 that provide a discharge path.Resistances between the bases of the parasitic transistors Q1, Q2 canprevent simultaneous turn-on of the transistors Q1, Q2. Therefore, itmay take a significantly long time for all of the transistors Q1, Q2 tobe turned on to provide a discharge path in response to an electricaltransient. In the structure illustrated, the turn-on time of all thebipolar transistors Q1, Q2 is generally dependent upon values ofon-resistance determined by dimensions of overlapped regions between thepolysilicon gates 3 and the drains D. However, as described above, arelatively small on-resistance may be provided by a transistorfabricated by a salicidation process, which may provide insufficient RCdelay time to enable a desired level of conduction of the transistorsQ1, Q2. This can result in insufficient ESD protection.

A protection circuit, such as a diode or silicon-controlled rectifier(SCR), may be used to provide ESD protection for an output drive circuitthat includes MOS transistors produced by a salicidation process. It isgenerally desirable to provide a protection circuit capable of driving alarge current at a relative low voltage, as it is generally desirablethat the protection circuit discharge excessive transients before theMOS transistors of the output drive circuit exhibit break down. However,it may be difficult to provide a protection circuit with high currentcapability at a relatively low turn-on voltage.

One way of dealing with this problem is to raise the turn-on voltage ofthe output drive circuit. For example, turn-on voltage may be raised byincreasing a base width of a parasitic LNPN bipolar transistorassociated with a MOS transistor of a drive circuit. However, increasingthe base width of a parasitic LNPN associated with an output drivecircuit MOS transistor can result in a need to increase circuit area tocompensate for lower current drivability.

FIG. 3 illustrates another way to increase the turn-on voltage of anoutput circuit 100 including a PMOS transistor 101 and an NMOStransistor 102 that drive an external signal pad PAD and are protectedby a protection circuit 20. As shown, turn-on voltage of the NMOStransistor 102 may be increased by placing a resistor Rs in series withthe NMOS transistor 102. This can restrain activation of a parasiticLNPN, but, as with extending base width, can lead to increased circuitarea to offset weak current drivability due to the additional resistor.

Another technique to protect a breakdown of an NMOS transistor byraising a turn-on voltage of an output circuit beyond that of aprotection circuit is shown in FIGS. 4A-4F. In particular, a base widthof a parasitic LNPN can be extended by connecting NMOS transistors of anoutput circuit 110 and a protection circuit 120 in series.

Two ways of connecting the transistors are shown in FIGS. 4B-4C andFIGS. 4D-4E, respectively. Referring to FIGS. 4B and 4C, active regionsof two NMOS transistors N1 and N2 are separated, and a source of theNMOS transistor N1 is connected by a metal line M to a drain of the NMOStransistor N2, which is grounded. FIGS. 4D and 4E show a configurationwhere the source of the NMOS transistor N1 and the drain of the NMOStransistor N2 are connected through an active region, which can moreefficiently use circuit area.

Although the configurations shown in FIGS. 4B-4E can extend a base widthbetween a drain connected to a pad PAD (i.e., a collector of theparasitic LNPN) and a source connected to the ground voltage Vss (i.e.,an emitter of the parasitic LNPN), these configurations may not providea desirable current gain because of presence of a parasitic bipolartransistor Q3 having an extended base width (see FIG. 4F). This can leadto poor ESD protection.

Other techniques for improving ESD protection for a salicidation MOStransistor are shown FIGS. 5A, 5B, 6A and 6B. In FIG. 5A, after formingN+ source and drain regions S, D in a substrate P-sub by means of an ionimplantation, a part of an insulation film 41 formed on top spaces ofthe polysilicon gate layer GP and source/drain regions is removed. Anopening 42 is then formed, exposing parts of the gate layer GP and thesource and drain regions S, D. After a local salicidation using theinsulation film as a mask, silicide films 44 are formed on the exposedsurfaces of the gate layer and the source/drain regions S, D as shown inFIG. 5B. The configuration shown in FIGS. 5A and 5B can provide basewidth extension, but may be difficult and/or expensive to manufactureand may not provide desirable high frequency operation due to increasedparasitic resistance.

Referring to FIGS. 6A and 6B, an NMOS transistor is fabricated byforming source and drain regions S, D in a substrate P-sub formingextended diffusion regions S′ and D′ under the source and drain regionsS and D by means of a high-energy ion implantation, and then performinga salicidation process. Although the double-diffused salicidationtransistor shown in FIG. 6B may have a wider discharge space due to theuse of the deep-extended diffusion regions S′ and D′, the transistor mayexhibit increased resistance and may require additional process stepsfor its fabrication. In addition, such a transistor may not havedesirable ESD protection.

SUMMARY OF THE INVENTION

According to some embodiments of the present invention, an outputcircuit of an integrated circuit device includes first and second MOStransistors including respective spaced apart pairs of source and drainregions in a substrate, arranged such that respective first and secondchannels of the first and second MOS transistors are laterally displacedwith respect to one another. The output circuit further includes anisolation region in the substrate, disposed between the first and secondMOS transistors. A first conductor connects the source region of thefirst MOS transistor to a power supply node. A second conductor connectsthe drain region of the first MOS transistor to the source region of thesecond MOS transistor. A third conductor connects the drain region ofthe second MOS transistor to an external signal pad of the integratedcircuit device.

In some embodiments, a surface of the source region of the first MOStransistor that faces the drain region of the second MOS transistor issmaller than a surface of the source region of the first MOS transistorthat faces the drain region of the first MOS transistor. The isolationregion may comprise at least one insulating region in the substrate,disposed between the first and second active regions. The isolationregion may further comprise a guard region having a higher degree of thesame conductivity type of the substrate, disposed between the first andsecond active regions and connected to the power supply node.

In further embodiments of the present invention, an output circuitincludes a first MOS transistor comprising a first source region in asubstrate, a first drain region in the substrate, and a first gateelectrode disposed therebetween that controls a channel extendingbetween the first source region and the first drain region. The outputcircuit further includes a second MOS transistor comprising a secondsource region in the substrate, a second drain region in the substrate,and a second gate electrode disposed therebetween that controls achannel extending between the second source region and the second drainregion, arranged such that the first drain region and the second drainregion are angularly disposed at first and second angles with respect tothe first source region. An isolation region is disposed in thesubstrate, between the second drain region and the first source region.A first conductor connects the first source region to a power supplynode A second conductor connects the first drain region of the first MOStransistor to the second source region. A third conductor connects thesecond drain region to an external signal pad of the integrated circuitdevice.

In still further embodiments of the present invention, an output circuitcomprises an isolation region in a substrate surrounding first andsecond active regions in the substrate. The circuit further includes afirst MOS transistor that comprises a plurality of source regions and aplurality of drain regions in the first active region and respectivegate lines on the substrate between respective pairs of adjacent ones ofthe source and drain regions of the first MOS transistor, the sourceregions of the first MOS transistor connected to a power supply node.The circuit also includes a second MOS transistor comprising a pluralityof source regions and a plurality of drain regions in the second activeregion and respective gate lines on the substrate between respectivepairs of adjacent source and drain regions of the second MOS transistor,the drain regions of the first MOS transistor connected to the sourceregions of the second MOS transistor and the drain regions of the secondMOS transistor connected to an external signal pad of the integratedcircuit device.

The first and second active regions may be arranged in a parallelfashion such that the source regions of the first MOS transistor arepositioned opposite the drain regions of the second MOS transistor andthe drain regions of the first MOS transistor are positioned oppositethe source regions of the second MOS transistor. The sides of the sourceand drain regions of the first MOS transistor that face the second MOStransistor may be narrower than adjacent sides of the source and drainregions of the first MOS transistor, and sides of the source and drainregions of the second MOS transistor that face the first MOS transistormay be narrower than adjacent sides of the source and drain regions ofthe second MOS transistor.

Related fabrication method embodiments are also described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are equivalent circuit diagrams of NMOS and PMOStransistors.

FIG. 1C is a sectional view of a MOS transistor fabricated by asalicidation process.

FIGS. 2A and 2B are planar and sectional view, respectively, ofinput/output transistors.

FIG. 3 is an equivalent circuit diagram of a conventional outputcircuit.

FIG. 4A is an equivalent circuit diagram of another conventional outputcircuit.

FIGS. 4B and 4C are planar and sectional views, respectively, of aconfiguration for the circuit of FIG. 4A.

FIGS. 4D and 4E are planar and sectional views, respectively, of anotherconfiguration for the circuit of FIG. 4A.

FIG. 4F is an equivalent circuit diagram showing parasitic bipolartransistors for the configurations of FIGS. 4B-4E.

FIGS. 5A and 5B show operations for fabricating an NMOS transistor witha conventional partial salicidation process.

FIGS. 6A and 6B show operations for fabricating an NMOS transistor witha conventional ion implantation technique.

FIG. 7 is a circuit diagram of a protected output circuit according tosome embodiments of the present invention.

FIGS. 8 and 9 are planar and sectional views, respectively, of aconfiguration of a protected output circuit according to someembodiments of the present invention.

FIG. 10 is an equivalent circuit diagram of parasitic bipolartransistors present in the structure shown in FIGS. 7-9.

FIGS. 11A-11C and 12A-12C illustrate output circuits according tovarious embodiments of the present invention,

DETAILED DESCRIPTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the thickness of layers and regions are exaggerated forclarity. Like numbers refer to like elements throughout. It will beunderstood that when an element such as a layer, region or substrate isreferred to as being “on” another element, it can be directly on theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly on” another element,there are no intervening elements present. Moreover, each embodimentdescribed and illustrated herein includes its complementary conductivitytype embodiment as well.

FIG. 7 shows an output circuit 11 of an integrated circuit device 700according to embodiments of the present invention, illustratingdischarge paths between an external signal pad PAD of the integratedcircuit 700 and a power supply voltage node VDD, and between theexternal signal pad PAD and a power supply ground voltage node VSS. Theoutput circuit includes first and second NMOS transistors N1, N2connected in series with a PMOS transistor P1 between a power supplyvoltage node VDD and a power supply ground node VSS. The output circuitis driven by signals PG, NG applied to respective gate electrodes of thePMOS transistor P1 and the NMOS transistor N2, which responsively drivethe external signal pad PAD of the integrated circuit device 700.

When a positive transient voltage is present between the external signalpad PAD and the power supply voltage node VDD, the positive transientmay be discharged to the power supply voltage node VDD through adischarge path PDp including a forward-biased P+/N junction at thesource of the PMOS transistor P1. When a negative transient voltage ispresent between the signal pad PAD and the power supply voltage nodeVDD, the negative transient may be discharged through two paths,including a primary discharge path through a forward-biased N+/Pjunction between the drain of the NMOS transistor N1 and the substrateof the integrated circuit 700, and a secondary discharge path PDnthrough a power protection circuit 60 connected between the power supplyvoltage VDD and the ground voltage VSS.

When a positive transient voltage is present between the signal pad PADand the power supply ground voltage node VSS, the positive transient maybe discharged through two paths, including a primary discharge paththrough a forward-biased P+/N junction of the PMOS transistor P1, and asecondary discharge path PSp through the power protection circuit 60 (adischarge path PSp). When a negative voltage transient is presentbetween the signal pad PAD and the power supply ground voltage node VSS,the negative transient may be discharged through a discharge path PSnincluding a forward-biased N+/P junction between the drain of the NMOStransistor N1 and the substrate.

FIGS. 8 and 9 are plan and sectional views, respectively, of an outputcircuit according to embodiments the present invention. In particular,FIGS. 8 and 9 illustrate a configuration that may be used to implementthe output circuit 11 of FIG. 7. FIG. 9 includes a section of the NMOStransistor N2, taken along a line Y1-Y1′, and a section of the NMOStransistor N1, taken along a line Y2-Y2′.

Referring to FIGS. 8 and 9, NMOS transistors N1 and N2 are formed infirst and second active regions 71, 72, which are separated from eachother by first and second field oxide insulation regions FOX. The firstactive region 71 includes drain regions D1, D2, and source regions S1,S2, S3. Gate electrodes 73, 74, 75, 76 are disposed between adjacentsource and drain regions. The second active region 72 includes drainregions D3, D4, D5 and source regions S4, S5, with gate electrodes 73′,74′, 75′, 76′ disposed between adjacent source and drain regions. Thedrain regions D1, D2 of the NMOS transistor N1 are connected to anexternal signal pad PAD through conductive layers (or lines) 77, 78, andthe source regions S4, S5 of the NMOS transistor N2 are connected to thepower supply ground voltage node VSS through conductive layers (orlines) 79. The gate electrodes 73, 74, 75, 76 of the NMOS transistor N1are connected to a power supply voltage node VDD, and the gateelectrodes 73′, 74′, 75′, 76′ of the NMOS transistor N2 are connected toan input signal node NG. The gates of the NMOS transistors N1, N2 may beformed, for example, by a salicidation process.

The source regions S1, S2, S3 of the first NMOS transistor N1 areconnected to the drain regions D3, D4, D5 of the second NMOS transistor72 through conductive layers 81, 82, 83. Avoiding connection of theseregions through a common diffusion region can help improve ESDprotection. For the illustrated embodiments, smaller (narrower) faces ofthe source regions S1, S2, S3 of the first NMOS transistor N1 and thedrain regions D3, D4, D5 of the second NMOS transistor N2 face oneanother, which can also reduce the effect of a parasitic NPN transistorassociated with these structures. As shown, a P-type (P+) guard ring GDmay surround the insulation regions FOX and may be connected the powersupply ground node VSS. Such a P-type guard ring GD can further reducethe effect of the parasitic bipolar transistor, as the guard ring GD candecrease base modulation of the parasitic bipolar transistor.

As shown in FIG. 9, parasitic NPN bipolar transistors Q1, Q2, Q3, Q4 areassociated with the transistors N1, N2. A base-emitter voltage of theparasitic transistor Q1 is VDD (Vin-0.6)-Vth, where Vth is a thresholdvoltage of the transistor N1. As the base to emitter voltage of thetransistor Q1 is higher than a turn-on voltage of the parasitic NPNtransistor, increasing base current for forward conduction may not causethe parasitic NPN bipolar transistor to be activated.

FIG. 10 shows an equivalent circuit for the structure of FIG. 9,including the parasitic transistors Q1, Q2 illustrated in FIG. 9, alongwith an additional parasitic bipolar transistor Q′ that includes acollector and emitter coupled to the signal pad PAD and the power supplyground voltage node VSS, respectively. The gain the transistor Q′ may belower than that of the parasitic transistor Q3 shown in FIG. 4F, becausethe opposing faces of the drain regions, D1 and D2, connected to theexternal signal pad PAD, and the source regions, S4 and S5, connected tothe power supply ground node VSS, can be made narrower than in theconventional configuration of FIGS. 4D-F. In contrast to the structurein FIG. 4D, the paths between these faces are angularly displaced from,i.e., not collinear with, the channels of the transistors N1, N2 (in theillustrated embodiment, the paths between these opposing faces areperpendicular to the channels of the transistors N1, N2). Interposingthe P-type guard ring GD between the source and drain regions of thefirst and second transistors N1, N2 can further reduce the gain of theparasitic bipolar transistor Q′.

Operations for fabrication the circuit of FIGS. 8 and 9 will now bedescribed. Referring to FIG. 9, a field oxide layer FOX is formed on theP-type substrate 900, defining the first and second active regions 71,72. Next, a gate oxide film (not shown) is formed on the active regions71, 72. A conductive material layer, e.g., a doped polycrystallinesilicon layer, is then formed on the substrate 900, and then patternedto form the gate electrodes 73, 74, 75, 76 of the first transistor N1and the gate electrodes 73′, 74′, 75′, 76′ of the second transistor N2.

N-type impurities are then implanted into the first and second activeregions 71, 72 to form the source and drain regions, S1, S2, S3, S4, S5,D1, D2, D3, D4, D5, using the gate electrodes 73, 74, 75, 76, 73′, 74′,75′, 76′ and the field oxide regions FOX as a mask. The guard ring GDmay be formed in the substrate 900 around the first and second activeregions 71, 72, by, for example, implanting P-type impurities into thesubstrate 900. After forming an inter-layer isolation film on thestructure with the first and second transistors, N1 and N2, theconductors 77, 78, 79, 80, 81, 83, 83 are formed on the insulationfilms. The conductors 77, 78, 79, 80, 81, 83, 83 are electricallyconnected to the source and drain regions, S1, S2, S3, S4, S5 and D1,D2, D3, D4, D5 and the guard ring GD through contact holes penetratingthe inter-layer isolation films.

As shown in FIGS. 11A, 11B and 11C, the gate electrodes of thetransistors N1, N2 may be connected in a number of different ways. Forexample, as an alternative to the connection shown in FIG. 11A, the gateelectrodes of both transistors N1 can be coupled to the signal inputnode, as shown in FIG. 11B, or coupled to different signal input nodesNG1, NG2, as shown in FIG. 11C.

It will be appreciated that the present application is also applicableto providing ESD protection between an external signal pad PAD and apower supply voltage node VDD, as shown in FIGS. 12A, 12B and 12C. Inthese embodiments of the present invention, structures complementary tothose in FIGS. 8 and 9 (in terms of conductivity type) may be used forPMOS transistors P1, P2. As shown in FIG. 12A, the gate electrode of thetransistor P1 may be connected to an input signal node PG, with the gateelectrode of the transistor P2 being connected to a power supply groundnode VSS or a reference voltage node Vref. In other embodiments, gateelectrodes of both transistors P1, P2 can be connected to an inputsignal node PG, as shown in FIG. 12B, or to separate input signal nodesPG1, PG2, as shown in FIG. 12C. It will be further understood that thepresent invention is also applicable to MOS transistors with gatesformed by non-salicidation processes, as activation of a horizontalbipolar loop can be restrained by forming double-diffused sources anddrains in an isolated diffusion region.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being set forthin the following claims.

1. A method of fabricating an output circuit for an integrated circuit,the method comprising: forming an isolation region in a substrate,defining first and second active regions in the substrate; forming afirst MOS transistor in the first active region; forming a second MOStransistor in the second active region, arranged such that a channel ofthe second MOS transistor is laterally displaced with respect to achannel of the first MOS transistor and not co-linear with the first MOStransistor; forming a first conductor on the substrate connecting asource region of the first MOS transistor to a power supply node of theintegrated circuit; forming a second conductor on the substrateconnecting a drain region of the first MOS transistor to a source regionof the second MOS transistor; and forming a third conductor connecting adrain region of the second MOS transistor to an external signal pad ofthe integrated circuit device.
 2. A method according to claim 1, whereina surface of the source region of the first MOS transistor that facesthe drain region of the second MOS transistor is smaller than a surfaceof the source region of the first MOS transistor that faces the drainregion of the first MOS transistor.
 3. A method according to claim 1,wherein the channels of the first and second MOS transistors aresubstantially parallel.
 4. A method according to claim 1: wherein thefirst MOS transistor comprises: a plurality of source regions in thefirst active region; a plurality of drain regions in the first activeregion, a respective one of the drain regions of the first MOStransistor disposed between first and second ones of the source regionsof the first MOS transistor; and a plurality of gate electrodes,respective ones of which are disposed between respective adjacent sourceand drain regions of the first MOS transistor; and wherein the secondMOS transistor comprises: a plurality of source regions in the secondactive region; a plurality of drain regions in the second active region,a respective one of the drain regions of the second MOS transistordisposed between first and second ones of the source regions of thesecond MOS transistor; and a plurality of gate electrodes, respectiveones of which are disposed between respective adjacent source and drainregions of the second MOS transistor.
 5. A method according to claim 1,wherein the substrate has a first conductivity type, wherein the sourceand drain regions have a second conductivity type, and wherein formingan isolation region further comprises forming a guard region in thesubstrate having a higher degree of the first conductivity type than thesubstrate, disposed between the first and second active regions andconnected to the power supply node.
 6. A method according to claim 5,wherein forming an isolation region comprises forming a first insulatingregion surrounding the first active region and a second insulatingregion surrounding the second active region, wherein forming a guardregion comprises forming a guard ring surrounding and separating thefirst and second insulating regions.
 7. A method according to claim 1,wherein the power supply node is a power supply ground node or a powersupply voltage node or a voltage reference node.